Disk drives are employed to store large amounts of information in bits encoded on tracks on the disk in a series of logical 1's and 0's. These logical 1's and 0's are represented in bit cells, which are areas of uniform size along the length of the track of the disk. It is desirable that the information bits be encoded on the disk as densely as practical so that a maximum amount of information may be stored. This can be achieved by increasing bit cell density on the disk, namely by reducing the size of bit cells along a particular track, thereby increasing the number of bit cells on the track. Increasing the number of bit cells per track increases the number of bits that can be encoded on each track, and therefore increases the amount of information stored.
Conventionally, logical 1's are recorded as transitions in magnetic flux on a magnetic disk for the given bit cell, and the absence of a transition indicates a logical 0. These transitions are created by switching the write current polarity through the write head. Transitions representing logical 1's are preferably placed within each bit cell near the center of the bit cell so that the data frequency (based on bit cell size and rotational speed of the disk can be accurately locked by a phase-locked loop during recovery of data from the disk and to ensure that bits are not encoded over a bit cell boundary during write operations. As bit cells are more densely packed on the track, placement of the transitions becomes even more important and difficult to precisely control. Thus, transition placement accuracy and bit cell density are two very important parameters in a write circuit for a disk drive.
Due to the inductive nature of a write circuit head and the output capacitances associated with the write circuitry, ringing effects occur in the write current signal which tends to delay the settling of the write current to its final DC value. These ringing effects adversely affect both transition placement and bit cell size. One option when ringing effects are present is to simply wait for the write current to settle to its final DC value and then enable the next transition for encoding a bit. This option means that bit cell duration must be increased to allow time for the write current to settle. While the accuracy of transition placement within bit cells in such a system will not be negatively affected by the ringing of the write current, the density of bit encoding by the write circuit is poor in comparison to desired goals. Another option when ringing effects are present is to switch the write current before it has settled to its final value. This approach maintains acceptable encoding density but results in decreased placement accuracy of bit encoding and hinders subsequent recovery of data from the disk. More particularly, if the write current has not fully settled from a prior transition, switching for the next transition might commence at different, uncontrolled, current levels, which results in sporadic placement of transitions in bit cells. Therefore, both options entail undesirable performance trade-offs where ringing effects are present.
One known solution to the ringing problem has been to connect a damping resistor across the terminals of the write head. The resistive damping reduces the settling time for the write current signal flowing through the head. However, resistive damping has several negative effects on the performance of the write circuit. Since some of the write current is diverted through the damping resistor, write current through the head is reduced. To achieve the desired value of write current through the head, more current must be generated to flow through both the head and the damping resistor. More importantly, the damping resistor slows the rise time for write current transitions. This can adversely affect bit cell density. While resistive damping does reduce settling time, the slower rise times may not be acceptable for high performance write circuits. Undershoot may also occur, which could result in loss of saturation of the head media or contribute to the problem of switching from uncontrolled current levels and result in sporadic bit placement in the bit cells. Thus, there is a need for a damping system in disk drive write circuitry which overcomes the problems of ringing and the shortcomings of resistive damping solutions.